JP2000-230853A discloses a known load detection device applying a piezoelectric effect, which includes a first piezoelectric element serving as a vibration generating portion, a second piezoelectric element serving as a vibration detecting portion which is disposed adjacent to the first piezoelectric element, a vibration detecting means, a vibration characteristics calculating means, and a load calculating means. According to the known load detection device, the vibration characteristics calculating means calculates characteristics of a vibration which is propagated by the vibration of the first piezoelectric element, and the load calculating means calculates the applied load on the basis of an output signal from the vibration characteristics calculating means.
According to the above-described known load detection device, two types of piezoelectric elements including the piezoelectric element serving as the vibration generating portion and the piezoelectric element serving as the vibration detecting portion are provided. According to another known load detection device, for example, disclosed in JP2006-258693A, a single piezoelectric element is commonly applied as the vibration generating portion and the vibration detecting portion. The load detection device disclosed in JP2006-258693A includes a piezoelectric element generating a vibration and detecting the vibration, a drive portion which outputs a drive signal for vibrating the piezoelectric element, a vibration detecting portion which extracts a detection signal from the piezoelectric element and detects a vibration characteristics value of the piezoelectric element on the basis of the extracted detection signal, and a calculation portion which calculates a load applied to the piezoelectric element on the basis of the vibration characteristics value from the vibration detecting portion. In those circumstances, a superimposed signal which is formed by superimposing the drive signal from the drive portion and the detection signal from the piezoelectric element is transmitted to the vibration detecting portion.
According to the known load detection devices in which the piezoelectric effect is applied, a change of the output signal from the piezoelectric element is susceptible not only to an influence of vibration characteristics of the piezoelectric element to which a load is applied by an object to be measured but also to an influence of characteristics of the object to be measured which is in contact with a pressure sensing surface of the piezoelectric element, particularly to an influence of an applied state of a pressure applied to the pressure sensing surface. A load detection performance is influenced to be unstable by the latter, depending on the characteristics of the object to be measured which is in contact with the pressure sensing surface of the piezoelectric element, particularly, depending on the applied state of the pressure applied to the pressure sensing surface. The foregoing drawback is caused to both structures, a structure in which separate piezoelectric elements have a vibration generating function and a vibration detecting function, respectively, and a structure in which a single piezoelectric element has a vibration generating function and a vibration detecting function.
A need thus exists for a load detection device which is not susceptible to the drawback mentioned above.